A variety of techniques are known to shield receivers from interference and jamming from neighboring frequencies. However, some of these include techniques that require redesign or adding or swapping of components that incur substantial replacement costs. The field is in dire need of techniques that can substantially reduce interferences in the environment without having to change the existing system and available as an ‘add on’ solution.
Often RF transmitters and receivers for cellular, entertainment, and navigation operate in dense and dynamic spectral environments with little frequency spacing between adjacent channels. When signals from neighboring channels are unintentionally picked up by a receiver, this creates noise and interference, degrading the performance of the receiver and the system. Due to this issue, filters to reject unwanted neighboring frequencies are an essential component of modern RF receivers. However traditional LC filters often do not have sufficient quality factors to reject unwanted signals which are very close in frequency. For many applications quality factor Q's greater than 102 are required. Filters employing electromechanical resonators such as quartz and LiNbO3 crystal resonators, Bulk Acoustic Wave (BAW) and Surface Acoustic Wave (SAW) resonators, can be manufactured with large quality factors (>104) and provide a solution to this problem but currently need to be integrated into the receiver before deployment. For many existing systems uninstalling or replacing the antenna and/or receiver is undesirable due to cost or efforts, demanding a High-Q retrofit filtering solution as new interference sources emerge.
Several patented technologies exist for reducing the gain of an antenna over a select range of frequencies, however these techniques all employ some form of LC resonance to create the filter properties, limiting the insertion loss and out-of-band rejection. These inventions are usually comprised of arrays of electrically conductive elements and/or packaged inductors and capacitors tuned to resonate to create band pass or band reject characteristics for incident radiation. The mechanism for creating these resonances is based on the storage of energy in inductive and capacitive components and is therefore limited by the previously mentioned quality factor constraints associated with LC resonators. Even in the field of metamaterials and Frequency Selective Surfaces (FSS), the techniques have relied solely on LC resonances, which due to their quality factors limit the achievable filter characteristics for these devices.